Expand Token implementation

- Remove Tokenizer interface and make Token a struct
- Add method to check if Token is still valid
- Add Token methods to write and read to file
- Set Token to api using token string
- Update documentation example
- Add Token constructor
- Fix typo in documentation
- Cleanup unused code
Version 0.4.0

This version basically brings a new Token implementation,
of its type itself and how the Api type deals with it.

It also fixes some typos in the documentation.
Merge pull request #2 from fern4lvarez/token

Cleanup Token implementation